Delta T (ΔT) is the difference between Terrestrial Time (TT) and Universal Time (UT1):
ΔT = TT – UT1
MICA determines the delta T value for a specific date and time by linear interpolation between adjacent entries in an internal table of historical and predicted values. The dates of the first predicted delta T values in each version of MICA are given in the following table.
| MICA Version | Date of First Predicted Delta T | |
| Julian Date | Calendar Date | |
| 2.0 | 2453279.50 | Oct. 1, 2004 |
| 2.1 | 2454040.50 | Oct. 31, 2006 |
| 2.2/2.2.1 | 2454863.50 | Feb. 1, 2009* |
To determine if the delta T file used by your copy of MICA is the most current, compare the Julian date of the first predicted delta T value with those in the table above. If the date matches the last date listed in the table, then you have the most current version. The date of first predicted delta T value is listed in "About Delta T" under the "Help" menu of a Windows system and in "About MICA" under the "MICA" menu of a Macintosh® system.
*This delta T file is included with the MICA 2.2.1 update on CD-ROM that is distributed by Willman-Bell, Inc. See the ordering information for further details. Long-term delta T information is also available from USNO's Earth Orientation Department and The Astronomical Almanac Online.
