Working with ages and date of birth

I'm new in ODK and I'm already pleased. I've been looking for an issue and I'm not realy satisfied of what I've found in the forum and elsewhere.
I have three variables date_obs: Date of observation; date_birth: Date of birth and age.
The individuals must have between 2 months and 5 years. When the exact date of birth is unknown, the clerk must mark '09-09-2099' and write the age, in month. I have writen the following on Excel:

date_obs constraint: . <= today()
date_birth constraint: ((. <= ${Q.104} - 60 ) and (.>= ${Q.104} - 1797 )) or (. = '9 sep 2099')
age constraint: (.>=2) and (.<=59)
age relevant: ${Q.113}!='09 sep 2099'
age default: (today() - ${q0113})/30.4375

I tried the date() and decimal-date() functions but these do not work on enketo. What's wrong with my data?

Date_Age.xls.xlsx (10.6 KB)